Thermoelectric compositions of ta w-se



United States Patent 3,197,410 THERMUELECTRIC COMPQSITIDNS 0F Ta W SeLothar H. Brixner, West Chester, Pa, assignor to E. I. du

Pont de Nernours and Company, Wilmington, Del, a

corporation of Delaware No Drawing. Filed Aug. 23, 1961, Ser. No.134,148

1 Claim. (Cl. 252-d2.3)

This application is a continuation-impart of my appli cation Serial No.846,461, filed October 14, 1959, and now abandoned, and Serial No.98,293, filed March 27, 1961, and now abandoned.

This invention relates to new thermoelectric materials. Particularly itrelates to new thermoelectric materials comprising at least onechalkogen selected from the group consisting of selenium and telluriumin combination with at least one metal selected from the groupconsisting of niobium and tantalum. In a preferred embodiment, itrelates to chalkogenides comprising at least one of the elementsselenium and tellurium in combination with niobium and with a heavymetal selected from the group consisting of tantalum, molybdenum andtungsten; or to a chalkogenide comprising at least one of the elementsselenium and tellurium in combination with tantalum and with a heavymetal selected from the group consisting of molybdenum and tungsten.

It is an object of this invention to produce solid-solution compounds ofthe general formula AB where A is a member selected from the groupconsisting of Nb Ta Nb Mo Nb W Ta Mo and Ta W where x is a positivenumber not greater than one, B is at least one chalkogen selected fromthe group consisting of selenium and tellurium; and z is a positivenumber not less than one and not greater than two.

It is also an object of the present invention to produce thermoelectricmaterials which are useful in devices where a Seebeck voltage(thermocurrent) is required.

t is a further object of this invention to produce thermoelectricmaterials which exhibit an exceptionally high Seebeck voltage atelevated temperatures, making these thermoelectric material-s especiallyvaluable for applications at such elevated temperatures.

The products of this invention can be prepared by firing under inertconditions a powdered mixture of the constituent elements in the ratiospecified by the general formula given above. The duration of the firingperiod will vary depending upon the reactants used, the size of thecharge, and the equipment used in firing. It may be stated as a generalrule that firing of the powdered mixture is continued until X-raydiffraction patterns fail to disclose the characteristic lines of thestarting components. The charge increases in volume during the firingand therefore the fired product should be pressed with or withoutcomminutions, and preferably reheated to sinter it into a compact massbefore being used in thermoelectric applications. The physicalproperties such as firmness and density are improved by the recompactingand subsequent sintering step. Pressure molding of the chalkogenide massprior to the sintering permits one to prepare shaped thermoelectricelements to fit the users needs.

The compositions of this invention are suitably prepared from theelements making up the composition, but

it is understood that one may start with chalkogenide compounds whichare blended with one or more other ingredients to form the desiredcomposition upon firing. The intimate mixture of the ingredients priorto firing is recommended so as to obtain uniformity in the product. Apreferred operation is to subdivide the product of the first firing andthen press the material into a mold of a desired shape in order toobtain the product ready for use in thermoelectric devices.

In a preferred method of preparation, the thermoelectric compositions ofthis invention are formed by grinding together the selected componentelements. The ground mixture is then compacted into pellets and fired inan inert atmosphere at temperatures in the range of 700 C. to 900 C. Thefired product is repressed into a desired shape and refired to sinterthe material into a strong, coherent end-product. It is preferred to usecommercially available reactants of the highest purity, and to have themin a -200 mesh (standard screen scale) particle size before firing. Theproducts are conductors of electricity, possess thermal stability attemperatures to at least 700 0, exhibit low thermal conductivity and alarge Seebeck etfect. Some of the products of this invention have beenfound to exhibit exceptionally high Seebeck coefficients at elevatedtemperatures. Seebeck coefficient is calculated from measured voltage ata known temperature differential by the formula E.M.F. AT

Also, all of the compositions have a figure of merit of at least 7.8 9C. at 35 C., and preferred compositions have a figure of merit of atleast 1 10 C. at 25 C. The figure of merit for thermoelectric materialtakes into account the fact that low resistivity and low thermalconductivity as well as high Seebeck coefiicient are necessary for agood thermoelectric material. The equation for calculating the figure ofmerit is a follows:

S2 l le Of merit 1n"C.* )=fic Example 1 This example describes thepreparation of TaTe. 9.0000 grams of Ta and 6.3459 grams of Te (TazTeatom ratio 1:1), both of -200 mesh, were mixed by ball milling in anagate mill, and the mixture compacted by a pressure of 20,000 p.s.i.into a 1" diameter pellet. This pellet was fired at 800 C. for 14 hoursunder Ti-Zr gettered argon. During this time, the volume of the pelletincreased about 4-fold. No weight loss was encountered. The shinycrystalline reaction product obtained was pressed into a bar A" x /4" x2 and refired at 800 C. for 12 hours under argon. Again, no weight losswas detected. The X-ray pattern of the product showed no lines of thecomponents, thereby indicating the formation of a new compound. Thecompound crystallized in the D space group, with parameters a =10.904 A.and C =20.102 A. The density of the bar was 8.04 gins. cm. Theresistivity of the bar was measured by the four-point method and wasfound to be 1.21 milliohm-cm. Electrical energy was developed by buttingthe bar between two copper blocks (machined from the same piece ofstock) maintained at different temperatures. Temperatures were measuredat approximately the cross-sectional center of the bar immediatelybehind the contact faces. With a temperature differential (AT) of 112 C.(T 28 C., T 140 C.), an of 13.5 millivolts was obtained. The Seebeckcoefficient,

AT X 1000) Example 2 This example describes the preparation of TaTe6.000 grams of Ta and 8.472 grams of Te (Ta: Te atom ratio 1:2) wereground together and tired at 800 C. under gettered argon for 14 hours asdescribed in Example 13. A considerable increase in volume resultedduring this firing, with no loss in weight. X-ray analysis of theproduct after heating showed neither of the original components to bepresent. The product crystallized in the D space group with latticeparameters a =1O.904 A. and c =20.075 A. The product Was again pressedand fired in the manner described in the first example, and acrystalline, gray reaction product was obtained. The thermalconductivity (k) for TaTe is 0.019 watt/ cm. C. Tests on the materialwere made in the manner described in Example 1. An of 2.153 millivoltswas obtained with a temperature differential of 124 C. (T 30 C.; T 154C.). The Seebeck coefficient was calculated from this data; this valueand other electrical properties are given in Table 2.

Example 3 pellet increased in volume during the reaction without weightloss. Again, X-ray analysis of the cooled product failed to show anylines of the original components. The compound crystallized in the Dspace group with lattice parameters a =10.904 A. and c =20.119 A. Thereaction product consisted of gray, metallic crystals. These crystalswere repressed into a bar A" x /4" x 2" and refired at 800 C. underpurified argon for another 12 hours. During this second firing process,the bar sintered without weight loss into a strong body with a metallicluster. Tests on the material were made in the manner described inExample 1. An of 2.68 millivolts was obtained with a temperaturedifferential of 116 C. (T 28 C.; T 144 C.). The Seebeck coefficient wascalculated from this data; this value and other electrical propertiesare given in Table 2.

4;, Example 4 This example describes the preparation of NbTe 3.00 gramsof Nb and 8.241 grams of Te (Nb:Te atom ratio 1:2) were reacted in themanner described in the previous examples, and again a gray, metallicshining product was obtained. The X-ray pattern of this product did notshow any lines of the original components, thus indicating the formationof a new compound. The compound crystallized in the D 5 space group withlattice parameters a =10.904 A. and c =19.888 A. Tests on the materialwere made in the manner described in Example 1. An of 1.12 millivoltswas obtained with a temperature differential of 50 C. (T 20 C.; T 70C.). The Seebeck coefficient was calculated from this data; this valueand other electrical properties are given in Table 2.

Example 5 Stoichiometric quantities of the elements niobium and seleniumfor the formation of NbSe 5.000 g. Nb and 8.500 g. selenium, were groundtogether and sealed in an evacuated quartz ampoule. The powders were ofthe highest purity commercially available and were of 200 (US. StandardSieve) particle size. The sealed quartz ampoule was heated at 1000 C.for 12 hours. The product was found to be a gray material of metallicluster. It was reground and pressed into a bar A" x A x 2", rescaled inan evacuated quartz ampoule, and heated again for 12 hours at 1000 C.The product was examined by X-ray procedures and was found to show nolines of the starting components. The product crystallized in the Dspace group With lattice parameters a =3.439 A. and c =12.998 A.

The bar so prepared was tested for electrical properties and these aregiven in Table 2 below.

Example 6 Following the same procedure as given in Example 1,stoichiometric amounts of tantalum and selenium according to the formulaTaSe 10.000 g. Ta and 8.731 g. Se, were heated in a sealed quartzampoule at 1000 C. for 12 hours. The product was a gray material ofmetallic luster of much the same appearance as the product of Example 1.This product was reground, pressed and reheated as in Example 1, andtested for electrical properties. These are given in Table 2 below.X-ray analysis of the product showed that it crystallized in the D spacegroup, with lattice parameters a =3.431 A. and c =12.737 A.

Example 7 Following the same procedure as given in Example 1, NbSe wasprepared by heating 8.0000 g. niobium and 6.7899 g. selenium in a sealedquartz ampoule at 1000 C. for 12 hours. The product was reground,repressed, and reheated for 12 hours at 1000 C., and was then tested forelectrical properties. These are reported in Table 2. X-ray analysis ofthe product showed that it crystallized in the D space group with thelattice parameters a =3.437 A. and c =13.030 A.

Example 8 Following the same procedure as in the previous example,stoichiometric quantities of tantalum and selenium, 10.0000 g. tantalumand 4.3650 g. selenium were combined to produce TaSe. The product wastested for thermal and electrical properties, and the results are givenin Table 2. X-ray analysis of the product showed that it crystallized inthe D space group with the lattice parameters a =3.425 A. and c =l2.746A.

Example 9 The product of Example 5 was prepared in the form of singlecrystalline platelets with crystal edges up to 1 cm. by a transportreaction carried out in a sealed quartz tube according to the followingreactions:

To effect these reactions, about 2 mg. I and g. NbSe (the product ofExample 5) which was ground to 200 mesh particle size, were charged intoone end of a quartz tube approximately mm. x 200 mm. and this tube wasevacuated and sealed. The end of the tube containing the NbSe was heatedto 900 C. and the other end was heated to 700 C. These conditions weremaintained for a period of 12 hours during which time a transportreaction occurred and NbSe deposited in the cooler end of the quartztube in the form of monocrystalline platelets. Structural data on thesecrystals were obtained by single crystal X-ray analysis, and results aregiven in Table 1, below. The identity of the transported crystals withthe product obtained in Example 5 was also established with powderpatterns.

Example 10 Using a procedure similar in all respects to that of Example9 except that the charge consisted of about 2 mg. I and 10 g. of TaSe(the product of Example 6), single crystalline platelets of TaSemeasuring up to 1 cm. on a side were formed. For parameters and spacegroup, see Table 1.

Example 11 Using the same procedure as in Example 9, a charge consistingof about 2 mg. of I and 10 g. of NbTe (the product of Example 4), wassealed in a quartz tube and heated. Single crystalline platelets of NbTemeasuring up to 1 cm. on a side were formed. Structural charac teristicsof these single crystals were determined by X-ray analysis and thisinformation is given in Table 1, below.

Example 12 Using the same procedure as in Example 9, but usingapproximately 2 mg. of I with 10 g. of the product of Example 2, singlecrystalline platelets of TaTe were formed. Structural characteristics ofthese single crystals were determined by X-ray analysis, and these aresummarized in Table 1, below.

TABLE l.-STRUCTURAL CHARACTERIZATION OF THE AB TYPE SELENIDES ANDTELLURIDES OF NIOBIUM AND TANTALUM For comparison with crystallographicdata obtained on the single crystals of the compounds listed in Table 1above, single crystals of the known compounds WTe WSe MoTe and MoSe wereprepared. Space groups for these were determined as follows: WTecrystallized in the D space group; WSe MoTe MoSe all crystallized in theD space group. It was found that WSe MoSe and MoTe crystallized in astructure which was isomorphic with TaSe The crystal parameters weredetermined to be as follows: WSe a =3.280 A., 0 12.950 A.; MoSe z a=3.288 A., 6 12.900 A.; MoTe a =3.517 A., c =l3.949 A. WTe crystallizedin the orthorhombic form with a =l4.028 A., b =3.495 A., and c =6.240 A.

Example 13 This example describes the preparation of the compound Ta NbTe 3.0 grams of -200 mesh Ta powder, 1.5408 grams of 200 mesh Nb powder,and 8.466 grams of 200 mesh Te powder (TazNbzTe mol ratio of 0.5:0.5:2)were carefully mixed and ball milled in a mechanical agate ball mill.The mixture was then pressed into a pellet 1 inch in diameter by apressure of 20,000 psi. The pellet was slowly heated to 800 C., at arate of approximately 50 C. per hour under a flow of gettered argon.This reaction temperature was held for 10-12 hours, and an expandedcrystalline reaction product was obtained which was cooled in thefurnace. No loss of weight was detected. This reaction product, whichwas already homogeneous, was again ball milled and then pressed into abar A x A x 2". This bar, of gray metallic appearance, was refired at800 C. for approximately 24 hours and again cooled in the furnace.Again, there was no weight loss encountered during the reaction. TheX-ray pattern of the product showed no lines of the components, therebyindicating compound formation. The resistivity of the bar was measuredby the four-point method and was found to be 0.89 rnilliohmcrn.Electrical energy was developed by butting the bar between two copperblocks (machined \from the same piece of stock) maintained at diiferenttemperatures. Temperatures were measured at approximately thecrosssectional center of the bar immediately behind the contact faces.An of 6.11 millivolts was obtained with a temperature differential of165 C. (T 39 C; T 204 C.). The Seebeck coefficient was calculated fromthis data; this value and resistivity data are given in Table 2.

Example 14 1.8000 grams Ta, 2.7737 grams Nb, and 10.159 grams Te werereacted in the manner described in Example 1 to form Nb Ta Te Tests onthe material were made in the manner described in Example 13. An E.M.F.of 6.1 7 millivolts was obtained with a temperature differential of 173C. (T 44 0.; T 217 (3.). The Seebeck coeificient was calculated fromthis data; this value and the resistivity measurement are given in Table2.

Example 15 4.0000 grams Ta, 0.6849 gram Nb, and 7.5252 grams Te werereacted in the manner described in Example 13 to form Nb Ta Te Thethermal conductivity (k) for TaTe is 0.031 watt/cm. C. By testing in themanner described in Example I, an EMF. of 11.32 miiiivolts was obtainedwith a temperature differential of 60 C. (T 60 (3.; T C.). The Seebeckcoefiicient was calculated from this data; this value and theresistivity measurement are given in Table 2.

Example 1 6 4.000 grams Ta, 2.0546 grams Nb, and 5.644 grams Te werereacted in the manner described in Example 13 to form Nb Ta Te. Tests onthe material were made in the manner described in Example 1. An EMF. of37.45 millivolts was obtained with a temperature differential of 216 C.(T 56 C; T 273 (3.). he Seebeck coefficient was calculated from thisdata; this value and resistivity data are given in Table 2.

7 Example 17 2.5000 grams Ta, 3.8522 grams Nb, and 7.0547 grams Te werereacted to form Nb 75Ta0 25TC- Tests on the material were made in themanner described in Example 13. An 'of 15.6 millivolts was obtained witha temperature differential of 104 C. (T 45 C.; T 192 C.). The Seebeckcoefficient was calculated from this data; this value and a resistivityfigure are given in Table 2.

Example 18 a 6.000 grams Ta, 1.0273 grams Nb, and 5.644 grams Te werereacted to form Nb Ta Te. Tests on the material were made in the mannerdescribed in Example 16. An of 12.66 millivolts was obtained with atemperature difierential of 110 C. (T 38 C., T 148 C.). The Seebeckcoefficient was calculated from this data; this value and resistivitydata are given in Table 2.

Example 19 Stoichiometric amounts of niobium, molybdenum, and telluriumaccording to the equation were processed and tested as given in Example13. amounts of reactants used were:

3.003 g. Nb 1.034 g. Mo 11.000 g. Te

Electrical properties of the product are given in Table 2. The singlephase nature of this product was established by X-ray analysis. Thecompound crystallizes in D type, with a =3.591 A. and c =13.738 A.

Example 20 The Using the same procedure as given for Example 13,stoichiometric amounts of niobium, molybdenum, and tellurium accordingto the equation:

Example 21 According to the reaction:

4Nb MO Te stoichiometric quantities of the components in powder form,weighed to the nearest milligram, were charged into a quartz ampoule.The amounts of the components used were 3.102 g. molybdenum, 1.001 g.niobium, 11.000 g. tellurium. The charged quartz ampoule was evacuatedmm. Hg) and sealed. It Was slowly heated to 900 C. and held at thistemperature for 14 hours. The ampoule was then cooled, and the productremoved and ground to pass at 100 mesh (US. Standard Sieve) screen. Thepowder was compacted by a pressure of 50 t.s.i. into a bar A" x A" x 2".The bar was, again sealed in a quartz ampoule and heated at 600700 C.for a period of 48 hours. At the conclusion of this time, the ampoulewas cooled, and the bar removed from the ampoule and tested forelectrical properties. The properties are given, together with those ofthe products of the other examples of the operation of this invention,in Table 2 which follows these examples. I E

The single phase nature of this product was established by determiningthe space group and parameters in which it crystallizes. The product ofthis example,

. 0.2s o.'15 2 crystallizes in the D space group with parameters a=3.557 A. and c =13.798 A.

Example 22 Stoichiometric amounts of niobium, molybdenum, and telluriumaccording to the equation were processed in the same manner as given inExample 13 above. The weights of reactants used were 8.000 g. Nb 3.3047g. Mo 10.987 g. Te

The product was tested and properties are given in Table 2 below.

Example 23 Stoichiometric amounts of niobium, molybdenum and telluriumaccording to the following equation, were processed in the same manneras described in Example 13, and the resulting bar was tested forelectrical properties. These are summarized in Table 2 below.

loNbg Moo gTeg the weights of reactants used were 0.350 g. Nb 3.253 g.Mo 9.6143 g. Te

Example 24 Following the same procedure as in the previous examples,stoichiometric quantities of reactants according to the equation wereprocessed to a bar which was tested for electrical properties. These aresummarized in Table 2 below. The amounts of reactants used were 0.200Nb,3.9243 g. Mo, and 10.9879 g. Te

Examples 25-28 In the same manner as is given in Example 13, productscomprising the elements niobium, tungsten, and tellurium of the generalformula Nb W Te were prepared and tested for electrical properties (seeTable 2). The amounts of the reactants used, which are in stoichiometricportions for the equations which were given were as follows:

Weights of reactants, grams Reaction 0.300 g. Nb Nb+9 l+20Tc- 10Nb,iW0.0Tez I 5.3440 g. w

*The single phase nature 01' this product was established by determiningthe space group and parameters in which it crystallizes. The product ofthis example, NDU25WUJ5T8Z crystallizes in the D21, space group withparameters a =l4.08l A., b0=3.495 A., and cu=6.270 A.

9 Examples 29-31 Weights of reactants, grams Reaction Ex. 30".-.Ta+MO+2Te- 2THOjTvIOUjTG 2.6316 g. Mo

7.000 g. Te 2.6577 g. Ta

7.500 g. To

The thermal and electrical properties of these materials are given inTable 2.

Examples 32-34 In the same manner as is given in Example 13, productscomp-rising tantalum, molybdenum, and tellurium of the general formulaTa l\ IO Te were prepared and tested for thermal and electricalproperties. The amounts of reactants used, which are in stoichiometricproportions for the equations given, were as follows:

Weights of reactants,

Reaction grams {5. g. Ta

Ta+Mo+4Te- 2Ta Moo,5Te2 2. 6720 g. Mo

7. 055 g. Te 3. 000 g. Ta

8. 4657 g. To

The thermal and electric properties of these materials are given inTable 2.

Examples 35-37 In the same manner as is given in Example 13, productscomprising tantalum, tungsten, and tellurium of the general formula Ta WTe were prepared and tested for thermal and electrical properties. Theamounts of reactants used, which are in the stoichiomet-ric proportionsfor the equations given, were as follows:

Weights of reactants, grams Reaction 3Ta+W+8Te- Tau, 5Wo z5Tez 1. 3556g. W

{4. 0000 g. Ta

The thermal and electrical properties of these materials are given inTable 2.

Example 38 This example describes the preparation of a solid-solutionthermoelectric material of chemical composition Ta W Se Te The procedureused in the preparation of this material was the same as that describedfor Example 13. The Weights of tantalum, tungsten, sele- Ilium, andtellurium used were as follows:

W=6.000 g. Ta=1.9 g. Se=5.1518 g. Te=2.7753 g.

The single-phase nature of this multi-component compound was establishedby X-ray analysis. The lines could be indexed in accordance with the Dstructure with a =3.367 A., and c =19.544 A. The electrical propertiesof the product of this example are given in Table 2.

Example 39 Using the same procedure as given in Example 13, athermoelectric material of chemical composition corresponding to theformula Ta W TeSe was prepared. The amounts of materials used in thepreparation of this composition were? Ta=3.9336 g. Te:1.5504 g.56:3.4344 g.

The electrical properties of this composition are given in Table 2.

To demonstrate the single phase nature of this thermoelectriccomposition, the lattice parameters were determined. These were found tobe a :3.437 A. c :l9.950 A.

The composition Ta W SeTe crystallizes in the structure D3015.

Example 40 In the same manner as is given in Example 13, atherrnoelectric material of composition Ta W Te Se was prepared byheating together W=2.0000 g. Ta=5 .9008 g. Te=8.3260 g. Se:1.7173 g.

The electrical properties of this solid-solution thermoelectric materialare given in Table 2, below.

The single phase nature of this composition was dem onstrated bydetermining its lattice parameters. These were found to be a =l0.793 A.0 :20.53 A.

This composition crystallizes in the Dgd structure which characterizesthe compound TaTe The compositions of Examples 38, 39, and 40 areillustrative of the fact that in the compositions of the general formulaAB described in this invention, where A is a pair of elements selectedfrom the group consisting of Nb Ta Nb Mo Nb W Ta Mo and Ta W where x isa positive number less than 1, and B is Se or Te or combination ofthese, substitutions of elements chosen from those listed are possibleboth in the cation as well as in the anion sites, these substitutionsoccurring either in the A and B sites, separately, or in both sitessimultaneously.

1 1 Examples 41-49 In the same manner as is given in Example 13 productscomprising columbium, molybdenum, and selenium; tantalum, molybdenum andselenium; and tantalum, tungsten, and selenium were prepared by usingstoichiometric quantities of the reactants according to the equationsgiven below. The products were tested for thermal and electricalpropertiesand the results are given in Table 2, below.

EX. 1.500 g. Mo 9.875 g. Se

The single phase nature of this product was established by determiningthe space group and parameters in which it crystallizes. The product ofthis example,

crystallizes in the D space group with parameters a =3.42 A. and 0:12.65 A.

8.230 g. Se

The single phase nature of this product was established by determiningthe space group and parameters in which it crystallizes. The product ofthis example,

crystallizes in the D space group with parameters 11,:3373 A. and c=19.021 A.

Ex. 43. 1--ib+3M+sse- N19 rtro se 1.290 g. Nb 4.000 g. Mo

8.778 g. Se

The single phase nature of this product was established by determiningthe space group and parameters in which it crystallizes. The product ofthis example,

crystallizes in the D space group with parameters a =3.30 A. and 6:12.90 A.

6.220 g. Ta 1.100 g. Mo 7.240 g. Se

The single phase nature of this product was established by determiningthe space group and parameters in which it crystallizes. The product ofthis example,

crystallizes in the D space group with parameters a =3.408 A. and c=19.132 A.

3.434 g. Se

The single phase nature of this product was established by determiningthe space group and parameters in which it crystallizes. The product ofthis example,

crystallizes in the D space group with parameters a =3.408 A. and c=19.126 A.

122 Ex. 48. Ta+W+4Se-+2Ta W Se 0.983 g. Ta 1.000 g. W 1 717 g. Se

The single phase nature of this product was established by determiningthe space group and parameters in which it crystallizes. The product ofthis example,

as oe z crystallizes in the D space group with parameters a =3.367 A.and c :19.186 A.

Ex. 49. Ta+3W+8Se 4Ta W Se 0.328 g. Ta 1.000 g. W

1.145 g. Se

The single phase nature of this product was established by determiningthe space group and parameters in which it crystallizes. The product ofthis example,

crystallizes in the D space group with parameters a =3.339 A. and 012.802 A.

TABLE 2.STRUCTURES, ELECTRICAL, AND THERMAL PROPERTIES OF SOLID-SOLUTIONTHERMOELECTRIC MATERIALS Resistiv- Seebeck ity in coeIIlcient ExampleComposition Space milliohmin micro- No. group cm. (room volts pertemper- C.

ature) 1. 21 121 0. 36 17 1. 50 23 0. 26 15 0. 35 12 0. 40 14 1. 30 24.7 1. 11 37 O. 89 37 0. 56 36 7. 1 188 18. 8 173 3. 9 106 2. 3 11-5 1. 1229 2. 78 60 16. 1 158 27. 0 200 43. 5 220 66. 5 334 0. 729 1. 10 40 1.00 86 1. 17 100 1. 25 18 1. 57 7 4. 1 29 7. 07 30 2. 09 60 30. 2 180 l.40 80 1. 32 72 0.25 0.75Te2 1.10 59 fibifi flfifi l .n 0o.5- 5 50T20,5W0.5S6T0 1. 6 30 40 Tfl0.75W0.2s 1 5S0 5. 1. 4 35Nbo.1sl\ii0u.zssez 3. 39 13 3. 30 36 69. 50 107 5. 1 124 14. 52 157 26.30 200 47 Ta0.75W0 z5S02 1. 20 48 Tau 5W0 5S62. 1. G0 10 49 TaMaWuJsSez2. 70 26 Examples 50-58 The solid solution compounds produced inExamples 19, 20, 21, 25, 26, 27, 41, 42, and 43 above were tested forthermal and electrical properties at elevated tempera- 13 tures. Theresults of these tests are given in Table 3, below. Values for thefigure of merit are also given.

.9. the die for a given press force. The rectangular bar had a density90% of theoretical, or, of the density deter- TABLE 3.THERMAL ANDELECTRICAL PROPERTIES OF SOLID SOLUTION COMPOUNDS AT ELEVATEDTEMPERATURE Thermal conductivity K., watts] deg. cm.

Composition Resistivity in milliohm-cm.

Seebeck coefficient in microvolts per Figure of merit N at measured..."(Example 50) d0.

Nbo .sMOmTGg (Example 51) N130 .znMouJs et (Example 52) OJsWo 25T6g(Example 53) Libs .5W0.5Tez (Example 5 1) onWonsTe;

(Example 55) NboasMonaasezm (Example 56) NbnMo0 Se (Example 57) NuasMoorss z (Example 58) Anti *Figure of merit calculation based on thethermal conductivity measured for Example 51.

'tBased on thermal conductivity measured for Example 53.

Examples 59-68 T he products set forth in Table 4 are representative ofmaterials having the composition Ta W Se where x is in the range of0.10-0.01. These products were prepared by charging the elements inpowder form into a quartz ampoule mm. D. x 200 mm. The ampoule wasevacuated and sealed off by fusion. The sealed ampoule and its contentswere then placed in a furnace and heated at a rate of 100 C. per hour to700 C., where the temperature was held for 10 to 14 hours. At the end ofthis period, a dark, free-flowing powder reaction product was obtained.After cooling, this product was remixed or homogenized by shaking theampoule vigorously, but without opening it. The ampoule and its contentswere then replaced in the furnace and heated gradually to 1000-1200 C.,and maintained at that temperature for 10 to 14 hours. At the end ofthis period, a highly crystalline, metallic-looking reaction product wasobtained in a free-flowing form. For characterizing measurements, thethermoelectric product was pressed into a bar as in Example 1. To formsuch a bar, a sufficient amount of the reaction product was weighed out,the weight determined from previous experience, and charged into arectangular cavity in a steel die, the cavity measuring 2" long by 4"wide. This die was placed on the bed of a press, with the long dimensionof the rectangular cavity in the horizontal plane, and the die plunger,measuring 2" long by wide brought down vertically with a pressure of to50 tons per sq. in. thus compressing the crystalline powder to arectangular bar measuring 2" long by A wide by A" thick, this thirddimension having been predetermined by the weight of reaction productcharged to mined by X-ray structural determinations. The electricalproperties, that is, the resistivity and Seebeck coefficient weremeasured in the direction of the long dimension of the rectangular bar,as in Example 1.

TABLE 4 Seebeck coeiiicient in microvolts per C. at

Resistivity Lattice n1 parameters" milliohrncm. at

Example Composition TaoJowmosezufl 1. 10 33 3. 301

0.5922 g. Ta 8.0000 g.W 7.3861 g.Se

Taunownmseanu 0.6842 g.Ta 8.0000 g.W 7.4664 g.Sc

fiomWonuseznu 0.327s g.Ta 8.0000 g.W 7.1553 g.Se

o.o2sWn.orssez 0.2269 g.Ta 9.0000 g.W 7.9250 g.Se

*All of the products in this table are single-phase solid solutionswhich crystallize in the Dam structure of Mosci.

Examples 69-70 The procedure and methods of measurement used in theseexamples were the same as those described in Examples 5968. Thermalconductivity measurements were also made as in Examples 50-58 by amodification of the method of Francl and Kingery, J. Am. Ceram. Soc. 37,80 (1954). This method requires a cylindrical specimen 0.75 in. D. X 1in. high which was prepared by pressing. A weighed amount of thefree-flowing reaction product was charged into a cylindrical cavity in asteel die, the die placed in a press with axis of the cylindrical cavityvertical, and the other member of the die brought down so as to compressthe reaction product into a 0.75 in. D. x l in. cylinder, the heightbeing predetermined by the weight of product charged to the die cavity.According to the Francl and Kingery procedure, the thermal conductivitywas measured in the direction of the cylinder axis. The data on thecompositions and their properties are given in Table 5.

formed directly by firing Element A with Element B in such proportionsthat there is present one atom of A for every one to two atoms of B. Forexample, when a mixture of tantalum and tellurium containing one atom oftantalum for every atom of tellurium is fired, there is formed thechemical compound, tantalum monotelluride (TaTe). In instances where theamount of selenium or tellurium used is such that z in the empiricalformula AB is not an integer, the product behaves according towell-known principles of solid state physics, to form a compound havinga defect structure. When a portion of the niobium or tantalum isreplaced by a substitute metal, as heretofore specified, the product ofthis invention is a solid solution of the telluride or selenide ofniobium or tantalum with the telluride or selenide of the substitutemetal. Generally speaking, the systems con taining two metals usuallyhave better thermoelectric properties than those made up of only onemetal in combination with selenium and/ or tellurium. Moreover,investigation has indicated that these better thermoelectric propertiesmay be obtained when as little as 5 atom per cent of the second metal ispresent. As previously mentioned, it is also possible to use acombination of selenium and tellurium in preparing the compositions ofthis invention. Such a combination forms solid solutions of telluridesand selenides of the metal or metals used.

The addition of doping agents, such as is practiced in semi-conductorfields of technology, may also be practiced in connection with thisinvention. These doping reagents are usually added in minute quantitiesand are useful in developing special effects, usually an enhancement ofthe value of the product insofar as its major property is concerned. Itis contemplated that such additions may be made to my thermoelectriccompositions, but the product will contain as essential elements atleast one metal selected from the group consisting of TABLE 5Resistivity in Seebeck Thermal Fig. of merit, milliohm-cm. coefiicient,conductivity, deg: ExlaImple Composition VJdegr watts/cm.- deg.-

25 C. 600 O. 25 0. 600 C. 25 0. 600 C. 25 C. 600 C.

69 Ta W sez 1.90 2.10 75 182 0.21 0.007 1.4110- 2.210-

O 2585 g Ta 8. 5000 g.W 7. 5241 g.Se 70 T3u mWn 9nSez 6. 90 7. 120 3470. 21 0.007 0.99-10' 2.3-10- 0. 4141 g.Ta 8. 0000 g.W 7. 2308 g.Se

' The compounds of Examples 69 and 70 are single-phase solid solutionswhich crystallize in the Don structure of MoSez.

flo.oaWo.e z 3. 292 12. 938 fiomWoseSe: 3. 288 12. 966

The examples given above are representative of compositions which may bemade according to this invention. However, many other variations arealso possible. For example, one might prepare a composition midwaybetween that of Examples 31 and 34 which would have the compositionrepresented by the empirical formula Ta Mo Te In a similar manner,compositions intermediate between those of other examples can also beprepared.

The products produced in this invention are the result of chemicalreaction. In the simplest embodiment of the invention, from thestandpoint of composition, the product may be represented by the formulaAB where A is either tantalum or niobium, B is either tellurium orselenium, and z is in the range of 1 to 2. Products of this formula arechemical compounds, and they are The structural parameters are asfollows niobium and tantalum, and at least one chalkogen from the groupconsisting of selenium and tellurium.

The compositions of this invention are useful in generating electriccurrent directly from thermal energy without reliance upon mechanicalparts. Thermoelectric generators are well known in the art, andthermoelectric elements of a size and shape to fit any particulargenerator can be prepared from the compositions of this invention bycompacting these compositions into the desired size and shape prior tofinal firing. Also, the compositions described herein can be used inthermoelectric cooling devices.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to said details except as set forth in the appended claim.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

Thermoelectric compositions consisting essentially of tantalum,tungsten, and selenium in the proportions represented by the formula TaW Se where x is in the range of 0.10-0.01.

References Cited by the Examiner FOREIGN PATENTS 5/58 Great Britain.

OTHER REFERENCES Chemical Abstracts, vol. 50, 1956, page 16250(e).

Kovba, L. M. (3-phase of the tantalum-tellurium systern), ibid. vol. 4,1959, pages 2820-2822.

Ukrainskiy, Yu. M., Novoselovia, A. V., and Simanov, Yu. P.:Investigation of the Tantalum-Tellurium System; (in Russian)Issledovaniye sistemy tantal-tellur., Zhurnal Neorganicheskoi Khimii,vol. 4, No. 1, 1959, pages 148-152.

WINSTON A. DOUGLAS, Primary Examiner.

0 JOHN H. MACK, Examiner.

